The hepatic portal vein is the venous trunk that carries nutrients-rich blood into the liver. It arises from the union of the splenic and the superior mesenteric vein, with the former draining the spleen and pancreas, and the latter the small intestine and colon respectively. Right before they join together to form a trunk, the splenic vein receives blood from the last portion of large intestine (descending colon and rectum) via the inferior mesenteric.
The hepatic portal vein conveys not only deoxygenated blood but also the nourishing byproducts of digestion into the liver, where they are metabolized by the hepatocytes mitochondria into vital products necessary for the maintenance of all the body tissues. What makes this vein different from the rest of the veins is the fact that it does not run straight into the venous bloodstream of the cava vein, but into the liver instead.
As it runs upwards, the portal vein takes a rather oblique course to the right of the porta hepatis, which is a transverse narrow opening located between the quadrate lobe and the caudate lobe. Thus, having entered the liver, this thick venous trunk splits into a left and a right branch, then into smaller ones, with each one of them dividing into even smaller venous branches until they become venules. This is how the portal vein ends up in the portal capillaries which penetrates throughout the whole hepatic parenchyma.
All three veins that drain into the liver (superior mesenteric, splenic, and hepatic portal) and their smaller branches are make up the portal system. We have to make it clear that the other veins the drain the liver into the cava vein are not part of the portal system.
Below, the portal vein with the rest of the veins that merge into it to make up the portal system.
